In the field of material classification, it has been shown that a material can be classified by a material property based on the bidirectional reflectance distribution function, or BRDF. The BRDF quantifies the angular dependence of reflectance as a function of the incident light direction (ωi) and viewing direction (ωo), with these directions often being defined with respect to the surface normal vector (n). Each direction (such as ωi or ωo) is itself ordinarily defined by an azimuth angle (φ) and a zenith angle (θ), therefore the BRDF as a whole is typically a 4-dimensional function that may be used to define a surface characteristic of a material.